Hour Glass Specialties

Glass is an oxymoron it is
hard
yet bends, strong but brittle. It can be clear or opaque. Itcan expand and shrink greatly with a change in
temperature
or maintain very littlechange in size. Some glasses are heavy and some are
significantly
lighter. Glass has a chemical structure that
resembles a box of springs tangled together. This issimilar to many plastics, and accordingly, glass has
some plastic properties. When glass isdrawn out very thin, its brittleness is reduced and the
resulting fiberglass can be bent. Glass resists most acids and
alkalis, with the exception of hydrofluoric acid which iswhat glass producers used to frost glass before the
EPA.
However, there are specialformulations of glass that will even dissolve in water,
but these are the exception and notthe norm. To produce glass, sand, lime,
and soda or pot ash are powdered mixed and fired. Thismay seem easy however in industry today many additives
are used to gain thecharacteristics desired for production. For instance,
if the glass is not to expand muchwhen heated an amount of borax may be added, or if the
bubbles from melting the batchneed to be removed sooner an amount of arsenic may be
added. Colors require specialformulations of the batch itself. As an example,
manganese
turns glass amethyst, but theshade of color from blue to red can be changed by
substituting
pot ash for soda ash. The firing of sand, lime, and
soda ash requires more heat than the melting of glass. Infact cullet (chunks of glass ) is added to batch (sand,
lime and soda) to help bring downthe necessary reaction temperatures. There are several
reasons for this. First batch is apowder with many small air pockets entrained in the
pile.
These pockets act as insulation.Second , when firing batch, the sand does not usually
melt, this would take extremetemperatures over 3000 degrees F. Instead the sand is
dissolved into the molten lime andsoda much like the amalgamation of silver into mercury
at room temperaturesThe heat necessary to fire batch is not easily
obtained.
A simple. fire of wood or coal willnot melt batch. Instead, like a blacksmith’s forge, air
must be forced into the fuel. Mostfurnaces are fueled with natural gas and an induced
airflow,
which is usually mixed in aventuri and introduced into the firebox through
an expanding hole, like a cone so that thegasses in the pipe are moving fast and the gasses at
the larger opening are moving slowerand ignite. This allows the fuel to burn in the firebox
but as the flame propagates back intothe pipe the velocity and temperature of the faster
moving
cold gasses prevent the flamefrom entering the pipe. Another method involves heating
the air before introducing it to thefuel. But since the air has enough temperature to
ignite
the gas, they must be mixed at thepoint that they enter the furnace. Another problem is once we
achieve
a 2500 degree F. flame, what do we keep it in.most metals melt at these temperatures. Steel doesn't
melt until 2800 degrees, but at 2500it will oxidize and turn to rust. Fireboxes used to
contain
these temperatures are built of aspecial brick, or refractory, generally composed of
alumina,
with an outer layer of lightspongy insulating brick. this outer layer does not
resist
the corrosive action of glass but isvery good at preventing the transfer of heat. The inner
layer of refractory is glass resistantbut does not stop heat transfer as well, and can be
made
of either tightly stacked blocksmortared in place or a castable refractory which is
tamped
into place and has no seams. The outer structure is then
built around the block box. Usually an angle iron frame withsprings is used at the corners which compresses the
outside
layer of blocks to hold themsnugly. This allows for expansion and contraction when
the temperature inside is changed.All of this is set on a flat steel table to make the
box opening a workable height.A door to cover the opening is usually made of hard
refractory
and doesn't completely sealthe opening to allow escape of the burnt gasses unless
a chimney or exhaust port isincluded in the design. The door can be located on the
side or top however the top locateddoor will release more hot blast than the side door.

With the gas prices increasing every
year many
people are trying to recycle the heat from the exhaust back into the
combustion
air. Large corporations use regenerators that heat refractory with the
exhaust
and then reverse the air flow and use the hot refractory to heat the
incoming
combustion air. I have come up with a design where I have a stainless
heat
exchanger mounted in the exhaust flow and I pump combustion air through
to the
burner where I introduce the fuel. The combustion air is now 400
degrees at the
inlet of the burner and my gas bill is between $200 and $300 less per
month. The
system cost me about $400 to build and has lasted 13 months of
operation (as of
2/05). Filling the tank can be done
with a batch or recycled glass cullet. There are many glasscompanies that sell their cullet, but this is usually
the leftovers from their work andcontains debris and impurities from their tools, which
necessitates inspection of the culletand removal of unwanted impurities before charging the
furnace. Batch is more timeconsuming. It needs to be measured, mixed, and requires
a hotter fire, but yields a finerglass and can be made for more exact purposes. Colorants can be added to
either
batch or cullet, and can be bought from chemicalsuppliers or colorant manufacturers. I recommend buying
from a colorant manufacturerbecause of the simplicity of use and the technical
assistance
available to help get thedesired result. Chemicals on the other hand do not come
with instructions. Care must beobserved when using these chemicals and respiratory
protection
is advised. Generallyspeaking, cobalt oxide makes blue, selenium makes a
pink
or amberina, copper oxidemakes green and blue, cerium with titanium can make
yellow,
and nickel makes ablue-violet in potash glass. Many chemicals have different
effects on different formulations of glass, and someof these chemicals are extremely potent. A glass blower
I knew tried to make blue fromcobalt oxide, but didn't know how much to put in his
100 pound tank. He thought that hecould put in a hand full and then see what shade of
blue
he had and add more if needed.After the first handful and some mixing, he found the
glass was black. He did not knowthat cobalt oxide is used at a proportion of 5000 to
1 for commercial purposes. His100 lbs. tank (1600 Oz.) needed less than one ounce to
get the desired result, but in hishand he could have had 2 or 3 ounces. Chemical colorants cannot be
fully described without a good understanding of glassformulation. For instance treating glass with gold is
known to turn it red, however this ismore true of lead glass ( where lead oxide is
substituted
for some of thesand) and tends to make flint glass brown. Also, there
is a long list of chemicals andminerals used as additives that also affect the glass.
These include alumina, feldspar,nepheline syenite, kyanite, kaolin, cryolite, antimony,
arsenous oxide, barium carbonate itsoxide and sulfate, borax, litharge, red lead,
fluorspar.,
bone ash, iron oxide, caustic potash,saltpeter, potassium carbonate, salt cake, zinc oxide,
and many more. When batch is fired the metals
tend to become oxides and give off their other radicals( carbonate, sulfate & sulfide, Etc.). These free
radicals then form gasses and as bubbleshelp mix and refine the batch as they rise through the
melt (see refiningbubbles). The
remaining metal oxides
then yield their own properties to the mix. These
properties,
like the list of ingredients, arenumerous and the effect of each is not linear because
some enhance the effect of others,and some limit the effects of others. Like the basic
sand lime, and soda mix where sandmakes the batch fluidize at higher temperature, lime
also increases viscosity but onlybelow red heat, as it approaches white heat lime
decreases
viscosity, soda tends todecrease viscosity and increase solubility, borax tends
to decrease thermal expansion andcontraction. The list is very long.